Superabsorbent polymer (hereinafter referred to as SAP) and superabsorbent material are not technical terms. A more precise designation for present hydrogel-forming SAP is ionic hydrocolloid. There is general understanding in industry that to be called superabsorbent, a material should imbibe, absorb or gel about 10 times its own weight of fluid and retain it under moderate pressure. The fluid is taken into the molecular structure and not simply contained in pores from which it could be expressed by squeezing. The hydrogels used in soft contact lenses, semi-permeable membranes and ion-exchange resins are not superabsorbent. Inorganic materials and polysaccharides, which can gel fluids upon manipulation of solutions, are not superabsorbents unless they can generate a gel when simply exposed to the fluid.
It is commonly thought that SAP was invented about 1970 by USDA researchers studying grafted starch, as described in Absorbency by P. K. Chatterjee, editor, pages 197-198, Elsevier, New York, N.Y., (1985). Absorbent or gelling polymers are actually much older. As early as 1952, carboxylic ion-exchange resins were known which swelled up to 500-fold in sodium bicarbonate, as described in British patent 719,330 to Bayer. Nevertheless, the application of these absorbent polymers in consumer products was not to be recognized until much later. In 1954, a technique was disclosed for making swellable polymers in saturated magnesium sulfate solution in U.S. Pat. No. 2,810,716 to Markus wherein it was suggested that such polymers could be used in drug delivery. U.S. Pat. No. 3,229,769 to Bashaw and Harper issued Jan. 18, 1966 disclosed a lightly crosslinked poly(potassium acrylate) for gelling water used in fire-fighting. In 1966, such gelling polymers were actively being promoted for use in disposable sanitary products and soon thereafter, U.S. Pat. No. 3,669,103 to Harper et al. disclosed various medical and personal care uses of acrylic acid and acrylamide-based gelling polymers. This is probably one of the first suggestions in the literature for the use of synthetic polymers in gelling (absorbing) aqueous body fluids.
Since this time period, many synthetic SAPs have been made which are directly descended from these early materials.
Present commercially available SAPs are typically crosslinked poly(acrylic acid) or acrylic acid grafted on starch. The carboxyl functionality is partially neutralized with sodium or potassium hydroxide. Some versions made for agricultural purposes, particularly in Japan, involve graft polymerizing acrylonitrile onto gelatinized starch followed by hydrolysis of the polyacrylonitrile to poly(acrylic acid-co-acrylamide). Although primarily acrylic acid-based SAPs are disclosed in conjunction with the present invention, it should be understood that other types of SAP may be contemplated for use.
In the solution polymerization of partially neutralized acrylic acid, aqueous sodium hydroxide is mixed with a water solution of acrylic acid. The crosslinker (a difunctional monomer) is added followed by the free radical polymerization initiator. This method yields a rubbery continuous gel which is dried and ground to the desired particle size.
The suspension or inverse emulsion technique involves dispersing aqueous monomer and crosslinker in a hydrocarbon diluent. Organic-soluble free radical initiators are usually employed. Suspension polymerization gives spherical particles with the size controlled by the type and amount of suspending agent. The water is azeotropically removed and the particles recovered by filtration.
Although the actual operating conditions used by current producers of SAP may vary, the patent literature teaches a broad range of temperature, concentrations, types of initiators and crosslinkers. Patents which disclose hydrogel-forming polymer compositions for use in absorbent structures are listed below:
U.S. Pat. No. 3,901,236 to Assarsson et al. PA1 U.S. Pat. No. 4,062,817 to Westerman PA1 U.S. Pat. No. 4,076,663 to Masuda et al. PA1 U S. Pat. No. 4,286,082 to Tsubakimoto et al. PA1 U.S. Pat. No. 4,340,706 to Obayashi et al. PA1 U.S. Pat. No. 4,473,689 to Login et al. PA1 U.S. Pat. No. 4,535,098 to Evani et al. PA1 European Patent 75,510 PA1 German Patent 3,313,344
The prior art has recognized the independent operating variables of gel stiffness and retention capacity, particularly, U.S. Pat. Re. 32,649 to Brandt et al., which stresses high retention capacity of the gel, once swollen, upon a subsequently applied load.
However, the prior art has not heretofore recognized the importance of providing absorbent structures having hydrogel-forming polymers capable of swelling against an applied restraining force. Instead, the prior art, particularly the aforementioned U.S. Pat. Re. 32,649, only discloses the ability of a gel particle to retain fluid under an applied pressure after the gel has been allowed to freely swell, that is, without a restraining force applied during swelling.